The outflow vessels of the chick heart, pulmonary arteries, aortic arch, and right and left brachiocephalic vessels, are derived from the six paired aortic arches. Previous experimental evidence suggests that blood flow is the mechanisms for determining which arch vessels will involute and which will remain patent. In these experiments aortic arch blood flow is altered by constricting the conotruncus in stage 16 to 26 chick embryos. The critical periods of the aortic arch development are determined by studying the pattern and severity of aortic arch malformations. The patterns of aortic malformations in the chick parallel are those seen in human congenital cardiac defects. These experiments identify intracardiac blood flow as an important morphogenic force in the development of the heart and great vessels.